The present invention relates to the recording of interference patterns into photosensitive media, and more particularly concerns an interferometric scanning system and a corresponding method for recording long gratings into such media.
Many optical elements may be fabricated by impinging a light beam onto a photosensitive medium to modify its optical, chemical or mechanical properties. A common application of such a technique is to make diffraction gratings for use in spectroscopic, meteorological and optical communication instruments as well as other types of systems having a use for such diffraction gratings. A very popular type of grating is the fiber Bragg grating used in optical telecommunication systems and devices.
Manufacture of gratings by interferometric methods has been well known for many years. The most common of such methods is by using two generally expanded collimated laser light beams incident on the photosensitive material. After exposure, the photosensitive material is properly processed to obtain a volume or relief structure grating, depending on the photosensitive material used.
This method has many drawbacks, especially for the manufacturing of large area gratings (over 50 mm maximum dimension). Since the laser beam usually has a gaussian intensity profile, it has to be sufficiently expanded in order to provide a quasi-uniform illumination over the entire recording area. Also, a larger beam means larger optics, which are more expensive to obtain in a defect-free form. The surface quality and cleanness of the required optics (mirrors, lens) is also very critical for the production of noise-free gratings. Even though noise-free gratings may be made using spatially filtered non-collimated diverging beams, the result usually gives a non-uniform pitch distribution over the grating length.
To eliminate the need for large collimating optics, U.S. Pat. No. 4,093,338 (Bjorklund et al.) and more recently 5,363,239 (Mizrahi et Mollenauer) both disclose a technique using small scanning laser beams. Using a beamsplitter and very flat mirrors, the beam is split into two beams, which are then redirected to interfere on the recording plane. By precisely moving the mirror laterally, it is possible to move the interference area over the whole recording surface without disturbing the interference pattern. For this technique to work efficiently, the moving mirror must move without any angular deviation and the many mirrors involved must all be of a very high flatness. To write very large gratings (10-20 cm long or more), many large high quality mirrors are needed which may significantly increase the costs of the setup.
Another technique developed by Asseh et al. (J. Light. Tech., Vol15, No8 1997 pp1419-1423) is based on multiple consecutive exposures for extending the length of the grating. Each sub-grating is produced by a two-beam interferometer over a small area. The recording plane is moved at a constant speed under the small interference area and the position is monitored using a high precision interferometer. The pulsed laser used is fired at specific positions to maintain the phase of the grating along the length of the grating. This technique is very flexible, does not need large optics, but requires high precision feedback electronics.
It is therefore an object of the present invention to provide a system and method for recording a long grating into a photosensitive medium that does not require either large collimating optics or high precision feedback electronics.
Accordingly, the present invention provides an interferometric recording system for recording interference fringes in a photosensitive medium extending in a recording plane.
The system includes light generating means for generating first and second coherent light beams, and a guiding assembly for respectively guiding the first and second light beams along first and second light paths. Both light paths lead to a recording location on the photosensitive medium. The first and second light beams generate the interference fringes at the recording location. The guiding assembly includes a delay mirror disposed in the first light path to reflect the first light beam. This delay mirror is interdependent with the photosensitive medium and forms a fixed angle xcfx86 with the recording plane.
The system according to the invention also includes translating means for translating the photosensitive medium and the delay mirror with respect to the first and second light paths. In this manner, the recording location moves along the photosensitive medium and the interference fringes move along the recording location, the angle xcfx86 being selected to operationally couple this moving of the recording location and interference fringes.
In accordance with an alternate embodiment of the invention, the system also includes a stabilizing mirror disposed in the second light path to reflect the second light beam. The stabilizing mirror is also interdependent with the photosensitive medium. The translating means translates this stabilizing mirror with respect to the first and second light path.
The present invention further provides a method for recording interference fringes in a photosensitive medium extending in a recording plane. The method includes the steps of:
a) generating first and second coherent light beams;
b) respectively guiding the first and second light beams along first and second light paths leading to a recording location on the photosensitive medium, the first and second light beams generating the interference fringes at the recording location, the first light beam being reflected on a delay mirror disposed in the first light path, the delay mirror being interdependent with the photosensitive medium and forming a fixed angle xcfx86 with the recording plane; and
c) translating the photosensitive medium and the delay mirror with respect to the first and second light paths, thereby moving the recording location along the photosensitive medium and moving the interference fringes along the recording location, the angle xcfx86 being selected to operationally couple said moving of the recording location and interference fringes.
Other features and advantages of the present invention will be better understood upon reading of preferred embodiments thereof with reference to the appended drawings.